When you use a sink in the public restroom, after using the toilet, and the other person at the sink only starts to use the hand soap after noticing that you've used it. They've been soap shamed because they are too dirty to use the soap out of habit.

Rudolf was sloshing his hands under the water, merely rinsing them off. Chaz came out of the bathroom and proceeded to soap up his hands and wash them properly. Rudolf was embarrassed for not using soap and immediately starting pumping soap into his hands. Rudolf felt soap shame.